In particular, such optical structures are used in photodetectors, in particular in quantum well photodetectors, known under the acronym QWIP (for Quantum Well infrared Photodetector), operating in the medium infrared, in order to improve their detectivity. More precisely, these optical structures are used therein to obtain a concentration of the field into the detecting active area, which can thus have substantially reduced dimensions, to match the field concentrating area. In practice, the active area of the photodetector is provided in the vicinity of the structure and focused on the field concentrating (or localising) area. As a result, the signal to noise ratio is notably improved with respect to photodetector devices having no such optical structures: since the active area is smaller, the photoelectric noise generated therefrom is decreased. The active area is adapted to the field concentrating area, and as a result, nothing is lost in terms of wanted signal.
Concentrating the luminous energy on a reduced photodetector surface obtained through these optical structures relies on exploiting localising phenomena of near field and on properties of some surface waves called surface plasmons.
In particular, such optical structures for photodetector are disclosed in the application FR 000314717. In an exemplary embodiment given in this application, a corresponding optical structure includes a transparent dielectric material in the spectral range of the optical radiation. This layer is etched so as to obtain a relief y=h(x,z) in an orthogonal reference frame 0xyz, invariant along 0z and variable along 0x. It is covered with a metal film. The profile in the plane 0xy is such that it can be defined by a mathematical function, which is the sum of two periodical functions f and g, with f representing the coupling function between the incident light on the photodetector and the electromagnetic field of the surface excited wave, of space period Λ and g representing the localising function of space period ½Λ and which has a fault in periodicity, located in the centre portion of the optical structure. The relative phase δΛ between f and g determines the ultimate coupling properties of the surface wave. Let be λ the average wavelength of the incident radiation, and n the average refractive index of the optical structure, then the period Λ is advantageously λ/2n for the incidence wave coupling to be optimum. Concentrating the field is achieved in an area around the fault in periodicity of the structure.
In one example, corresponding optical structures have a stepwise or facet profile. The profile is obtained by means of several etching steps, whose number depends on the complexity of the profile to be produced.
There are technological problems with respect to the etching steps required to produce the optical structure disclosed in this application. In particular, this results in problems of re-alignment of the etching masks between each step and of checking the etching depth, that is step height, which is one of the most difficult technological parameters to be controlled.
Further, these optical structures provide low flexibility as to setting the localising properties of the electromagnetic wave under the optical structure. In particular, decoupling the wave decay along the 0x axis and along the 0y axis is unknown, because the only degree of freedom in the structure is the etching step duration. In the optical structures obtained according to that principle, the relative phase between both superposed networks is fixed.